Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. One system uses stretch wrapping machines to stretch, dispense and wrap stretch packaging material around a load. Stretch wrapping can be performed as an inline, automated packaging technique that dispenses and wraps packaging material in a stretch condition around a load on a pallet to cover and contain the load. Pallet stretch wrapping, whether accomplished by a turntable, rotating arm, vertical rotating ring, or horizontal rotating ring, typically covers the four vertical sides of the load with a stretchable packaging material such as polyethylene packaging material. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.
Stretch wrapping machines provide relative rotation between a stretch wrap packaging dispenser and a load either by driving the stretch wrap packaging dispenser around a stationary load or rotating the load on a turntable. Upon relative rotation, packaging material is wrapped on the load. Rotating ring style stretch wrappers generally include a roll of packaging material mounted in a dispenser, which rotates about the load on a rotating ring. Wrapping rotating rings are categorized as vertical rotating rings or horizontal rotating rings. Vertical rotating rings move vertically between an upper and lower position to wrap packaging material around a load. In a vertical rotating ring, as in turntable and rotating wrap arm apparatuses, the four vertical sides of the load are wrapped, along the height of the load. Horizontal rotating rings are, stationary and the load moves through the rotating ring, usually on a conveyor, as the packaging material dispenser rotates around the load to wrap packaging, material around the load. In the horizontal rotating ring, the length of the load is wrapped. As the load moves through the rotating ring and off the conveyor, the packaging material slides off the conveyor (surface supporting the load) and into contact with the load.
Historically, rotating ring style wrappers have suffered from excessive packaging material breaks and limitations on the amount of containment force applied to the load (as determined in part by the amount of pre-stretch used) due to erratic speed changes required to wrap “non-square” loads, such as narrow, tall loads, short, wide loads, and short, narrow loads. The non-square shape of such loads often results in the supply of excess packaging material during the wrapping cycle, during time periods in which the demand rate for packaging material by the load is exceeded by the supply rate of the packaging material by the packaging material dispenser. This leads to loosely wrapped loads. In addition, when the demand rate for packaging material by the load is greater than the supply rate of the packaging material by the packaging material dispenser, breakage of the packaging material may occur.
When stretch wrapping a typical rectangular load, the demand for packaging material varies, decreasing as the packaging material approaches contact with a corner of the load and increasing after contact with the corner of the load. When wrapping a tall, narrow load or a short load, the variation in the demand rate is even greater than in a typical rectangular load. In vertical rotating rings, high speed rotating arms, and turntable apparatuses, the variation is caused by a difference between the length and the width of the load. In a horizontal rotating ring apparatus, the variation is caused by a difference between the height of the load (distance above the conveyor) and the width of the load.
The amount of force, or pull, that the packaging material exhibits on the load determines how tightly and securely the load is wrapped. Conventionally, this force is controlled by controlling the feed or supply rate of the packaging material dispensed by the packaging material dispenser with respect to the demand rate of packaging material required by the load. Efforts have been made to supply the packaging material at a constant tension or at a supply rate that increases as the demand rate increases and decreases as the demand rate decreases. However, when variations in the demand rate are large, fluctuations between the feed and demand rates result in loose packaging of the load or breakage of the packaging material during wrapping.
The wrap force of many known commercially available pallet stretch wrapping machines is controlled by sensing changes in demand and attempting to alter supply of packaging material such that relative constant packaging material wrap force is maintained. With the invention of powered pre-stretching devices, sensing force and speed changes was immediately recognized to be critically important. This has been accomplished using feedback mechanisms typically linked to or spring loaded dancer bars and electronic load cells. The changing force on the packaging material caused by rotating a rectangular shaped load is transmitted back through the packaging material to some type of sensing device which attempts to vary the speed of the motor driven pre-stretch dispenser to minimize the force change on the packaging material incurred by the changing packaging material demand. The passage of the corner causes the force on the packaging material to increase. This increase force is typically transmitted back to an electronic load cell, spring-loaded dancer interconnected with a sensing means, or by speed change to a torque control device. After the corner is passed the force on the packaging material reduces as the packaging material demand decreases. This force or speed is transmitted back to some device that in turn reduces the packaging material supply to attempt to maintain a relatively constant wrap force.
With the ever faster wrapping rates demanded by the industry, the rotation speeds have increased significantly to a point where the concept of sensing demand change and altering supply speed is no longer effective. The delay of response has been observed to begin to move out of phase with rotation at approximately 20 RPM. The actual response time for the rotating mass of packaging material roll and rollers approximating 100 lbs must shift from accelerate to decelerate eight times per revolution that at 20 RPM is a shift more than every ½ sec.
Even more significant is the need to minimize the acceleration and deceleration times for these faster cycles. Initial acceleration must pull against the clamped packaging material, which typically cannot stand a high force especially the high force of rapid acceleration that cannot be maintained by the feedback mechanisms described above. Use of high speed wrapping has therefore been limited to relatively lower wrap forces and pre-stretch levels where the loss of control at high speeds does not produce undesirable packaging material breaks.
Packaging material dispensers mounted on horizontally rotating rings present additional special issues concerning effectively wrapping at high speeds. Many commercially available rotating ring wrappers that are in use depend upon electrically powered motors to drive the pre-stretch packaging material dispensers. The power for these motors must be transmitted to the rotating ring. This is typically done through electric slip rotating rings mounted to the rotating ring with an electrical pick up fingers, mounted to the fixed frame. Alternately others have attempted to charge a battery or run a generator during rotation. All of these devices suffer complexity, cost and maintenance issues. But even more importantly they add significant weight to the rotating ring which impacts its ability to accelerate and/or decelerate rapidly.
Packaging material dispensers mounted on vertically rotating rings have the additional problem of gravity forces added to centrifugal forces of high-speed rotation. High-speed wrappers have therefore required expensive and very heavy two part bearings to support the packaging material dispensers. The presence of the outer race on these bearings has made it possible to provide a belt drive to the pre-stretch dispenser. This drive is taken through a clutch type torque device to deliver the variable demand rate required for wrap force desired.
Accordingly, it is an object of the present invention to provide a method and apparatus for regulating the feed of packaging material to produce a secure load for shipment without distorting the top layers of a load, crushing product, or breaking film.
It is another object of the present invention to provide a method and apparatus capable of regulating the packaging material supply rate to maintain a wrapping force below the force that will incur film breaks.
It is an additional object of the present invention to provide a method and apparatus for wrapping loads at faster wrapping rates.
It is an additional object of the present invention to provide a method and apparatus capable of minimizing packaging material dispenser acceleration and deceleration times, in order to obtain faster wrapping cycles.
It is an additional object of the present invention to provide a method and apparatus that reduces the amount of complexity, cost, weight, and maintenance associated with known rotating ring apparatuses.